This invention relates to ring oscillators, and more particularly, to ring oscillators that are compensated for the effects of voltage, temperature, and/or semiconductor processing variables.
Ring oscillators are well known in the art. An odd number of series-connected inverter stages has the output of the last inverter stage coupled to the input of the first inverter stage to create an oscillation. The number of inverter stages determines the frequency of oscillation. Typically, each inverter stage has a first supply node connected to a first supply voltage such as five volts, and a second supply node connected to a second supply voltage such as ground. The generated oscillating signal can be used as a clock signal or the like. However, the operating frequency of the oscillating signal is unstable. Frequency can vary greatly as a function of voltage, temperature, and, if the ring oscillator is fabricated as an integrated circuit, semiconductor processing variables. Because of the frequency instability, ring oscillators are of relatively limited use in applications where a stable frequency is desired.
In the past, techniques for compensating integrated circuit ring oscillators have involved complex circuitry such as in U.S. Pat. No. 4,7l4,901 to Jain et al or separate circuits such as in U.S. Pat. No. 4,547,749 to Kuo. Such circuits, while providing a compensated ring oscillator having a stable frequency of oscillation, undesirably increase the area of the integrated circuit.
What is desired is a simple circuit including as few components as possible for stabilizing the frequency of oscillation of a ring oscillator with respect to voltage, temperature, and semiconductor processing variables.